Water-soluble cationic resins are often used as wet-strength additives in papermaking. One widely used type of wet-strength resin is the polyamide/polyamine/epichlorohydrin material sold under the trade name KYMENE. See, for example, U.S. Pat. No. 3,700,623 to Keim, issued Oct. 24, 1972; and U.S. Pat. No. 3,772,076 to Keim, issued Nov. 13, 1973. Another group of water-soluble cationic wet-strength resins are the polyacrylamides sold under the trade name PAREZ. See, for example, U.S. Pat. No. 3,556,932 to Coscia et al, issued Jan. 19, 1971; and U.S. Pat. No. 3,556,933 to Williams et al, issued Jan. 19, 1971.
The cellulosic fibers used in papermaking are negatively charged. Since the water-soluble wet-strength resins are cationic (positively charged), they are deposited and retained well when directly added to the aqueous pulp slurry. Such "wet-end addition" is highly desirable in papermaking. Subsequently in the papermaking process, these resins cross-link and eventually become insoluble in water. When this occurs, the wet-strength resin acts as a "glue" to hold the fibers of the paper together. This results in the desired wet-strength property.
Paper products made with such resins often have a stiff, paper-like feel. To impart greater softness to the paper product, styrene-butadiene latexes can be used as the binder system. However, these styrene-butadiene latexes are usually either nonionic in character or else are partially anionic due to inclusion of anionic comonomers or surfactants. The nonionic styrene-butadiene latexes cannot be used as "wet-end additives" in a conventional papermaking process. Instead, these nonionic latexes have to be impregnated or pattern-printed on the subsequently laid paper furnish, such as by the process described in European Patent application 33,988 to Graves et al, published Aug. 19, 1981.
An anionic styrene-butadiene latex can be used in a conventional wet-end additive papermaking process by adding a cationic polyelectrolyte. See, for example, U.S. Pat. No. 4,121,966 to Amano et al, issued Oct. 24, 1978; and U.S. Pat. No. 2,745,744 to Weidner et al, issued May 15, 1956. The cationic polyelectrolyte used is typically a water-soluble cationic wet-strength resin. Basically, the cationic polyelectrolyte, when added, destabilizes the dispersed anionic latex particles which then flocculate and deposit on the paper fibers. Accordingly, the cationic polyelectrolyte and anionic styrene-butadiene latex cannot be combined together until the point at which they are used as the binder system in papermaking.
Styrene-butadiene latexes have also been modified to provide cationic groups chemically bound on the surface of the latex particles. See, for example, U.S. Pat. No. 4,189,345 to Foster et al, issued Feb. 19, 1980; and U.S. Pat. No. 3,926,890 to Huang et al, issued Dec. 16, 1975. Incorporation of the cationic groups on the surface of the latex particles converts the latex into a wet-end additive like the water-soluble cationic wet-strength resins. These cationic latexes appear to have adequate colloidal stability, especially when nonionic or preferably cationic surfactants are added. However, the deposition and retention of the cationic latex particles on the paper fibers does not appear to be very great. Indeed, the cationic latex of the Foster et al patent appears to require a co-additive to enhance the deposition of the latex particles on the paper fibers.
Accordingly, a cationic latex which combines: (1) colloidal stability; (2) enhanced deposition and retention of the latex particles on the paper fibers; and (3) enhanced wet-strength properties, would be highly desirable.
The polycationic latexes of this invention provide these desirable benefits.
Despite the various art-described attempts to improve wet-strength resins, the wet-strength resin of choice has remained the polycationic material, KYMENE. Unfortunately, as noted hereinabove, the use of excessive amounts of KYMENE can cause paper treated therewith to become not only stronger, but also stiffer, which is undesirable for some uses. Stated otherwise, KYMENE not only enhances the wet tensile strength of the paper, but also increases its dry tensile strength, thereby leading to a stiff or brittle feel. This is undesirable in situations where paper with a soft, more cloth-like feel is desired.
Moreover, it has now been determined that KYMENE-type polycationic water-soluble wet-strength resins can undesirably interact with anionic additives which the formulator may wish to incorporate into the paper. For example, various anionic superabsorbent materials have their absorbency undesirably lessened when KYMENE is present.
In the present invention, it has been discovered that KYMENE-type wet-strength resins can be effectively rendered water-insoluble, and thus rendered less reactive to anionic paper additives. Moreover, it has been discovered that the polycationic latexes of the present invention desirably enhance the wet-strength of paper treated therewith, but without causing the paper to have an undesirable stiff feel. In addition, the maximum wet strength obtained with KYMENE seems to peak at about 150 g/in (for Northern Softwood Kraft handsheets) whereas the polycationic latexes herein can yield wet strengths as high as 1200 g/in. These and other advantages of the present invention will be appreciated from the disclosure hereinafter.